The invention relates to a process for producing an iron melt, wherein iron ore is reduced to sponge iron in a direct reduction zone, the sponge iron is melted in a meltdown gasifying zone while supplying carbon-containing material under gasification of the carbon-containing material to reducing gas and under formation of a slag and the reducing gas is injected into the direct reduction zone, is reacted there and is drawn off as a top gas, the reducing gas and/or the top gas being subjected to gas scrubbing and the sludges separated during scrubbing being mixed with binder and coal dust and subsequently agglomerated.
A process of this type is known from AT-B-376,241. There, the reducing gas, after the separation of solids, as well as the top gas emerging from the direct reduction zone are subjected to gas scrubbing in cyclones and the sludges separated are mixed with binder comprised of iron oxide dust, hard pitch, bitumen or bituminous brown coal, are hot-briquetted and are supplied to the meltdown gasifying zone, the iron oxide dust coming from a blast furnace gas purification arrangement.
The solids incurring in solids separation--primarily dust coal--are recycled to the lower region of the meltdown gasifying zone almost entirely; a small portion of the dust coal is mixed with the sludges mingled with binder and is briquetted together therewith.
In doing so, it is disadvantageous that, due to the elevated amount of iron oxides introduced, reduction work has to be done in the melter gasifier in order to reduce the iron oxide, energy required for the melting procedure, thus, being withdrawn from the same and the process occurring within the meldown gasifying zone being disturbed. Moreover, hot-briquetting is an expensive solution in terms of investment and operational costs.
From DE-A-41 23 626 it is known to agglomerate metallurgical residual substances by using binding agents, slag formers and reducing agents and to introduce the agglomerates into the upper burden region of a melting aggregate, preheating and drying of the agglomerates taking place in this burden region of the melting aggregate. The burden passes through the melting aggregate according to the counterflow principle, at first reaching a reduction zone provided within the melting aggregate and subsequently being melted in the lower region of the melting aggregate. This known process requires much energy inasmuch the waste and/or residual substances must be dried and sintered in the melting aggregate, thus adversely affecting the process that takes place within the melting aggregate.
Also in this case, reduction work must be done in the melter gasifier on account of the introduction of iron oxides (such as, e.g., scales), which additionally calls for a considerable amount of energy. Furthermore, it is proposed to use, for instance, sulfite liquors as binders, which, however, results in an undesired introduction of sulfur into the process.
The invention aims at avoiding these disadvantages and has as its object to supply back to the direct reduction process the sludges incurring in the direct reduction of iron ore in amounts as large as possible, i.e., in amounts of approximately 90% of the total amount of sludges, without influencing the process course of the direct reduction in any way. In particular, no changes in the amounts of raw materials and fluxes are to be necessary.
In accordance with the invention, this object is achieved by the combination of the following measures:
that the sludges separated during scrubbing are dehydrated to a residual moisture content prior to being further treated,
that, after this, coal dust and, as a binder, quick lime are admixed to the sludges,
that the sludges subsequently are granulated, and
that the granulate thus formed is supplied to the meltdown gasifying zone while increasing the basicity of the slag to a maximum of 1.25, preferably to a maximum of 1.20.
Preferably, the sludges are dehydrated to a residual moisture content of 25 to 50%, preferably 35 to 40%, before further treatment. Thereby, sludge having such a residual moisture content can be directly charged into a mixer granulator. The preferred range of residual moisture content results in a quantitatively favorable consumption of quick lime. Without such dehydration, a considerably higher consumption of quick lime would be necessary to obtain a sufficient strength of the granulate.
Suitably, coal dust is added to the dehydrated sludges in an amount ranging up to 30%, preferably up to 25%, of the amount of dehydrated sludge. Coal dust in the amount mentioned positively influences the strength of the granulate. The use of coal dust from dedusting arrangements of a coal drying installation, which is incorporated in the present concept of producing an iron melt or pig iron and/or steel pre-products, is particularly advantageous for reasons of environmental protection, avoiding transportation and dumping costs for the coal dust. Furthermore, the addition of coal dust to the dehydrated sludge has the advantage that the granulates subsequently formed within the mixer granulator have a considerable carbon content, the coal dust positively contributing to the energy supply as an energy carrier after having charged the granulates into the meltdown gasifying zone.
According to a preferred variant, the granulates are provided with a carbonate layer by exposure to a CO.sub.2 -containing smoke gas before being supplied to the meltdown gasifying zone. Such a carbonate layer increases both the strength for transportation and the disintegration strength within the meltdown gasifying zone.
Suitably, the formation of a carbonate layer for the granulates is realized in a coal drying arrangement.